Quantitative Buffy Coat (QBC) analysis is routinely performed in clinical laboratories for the evaluation of whole blood. The buffy coat is a series of thin, light-colored layers of white cells that form between the layer of red cells and the plasma when unclotted blood is centrifuged or allowed to stand.
QBC analysis techniques generally employ centrifugation of small capillary tubes containing anticoagulated whole blood, to separate the blood into essentially six layers: (1) packed red cells, (2) reticulocytes, (3) granulocytes, (4) lymphocytes/monocytes, (5) platelets, and (6) plasma. The buffy coat consists of the layers, from top to bottom, of platelets, lymphocytes and granulocytes and reticulocytes.
Based on examination of the capillary tube, the length or height of each layer is determined during the QBC analysis and converted into a cell count, thus allowing quantitative measurement of each layer. The length or height of each layer can be measured with a manual reading device, i.e., a magnification eyepiece and a manual pointing device, or photometrically by an automated optical scanning device that finds the layers by measuring light transmittance and fluorescence along the length of the tube. A series of commonly used QBC instruments are manufactured by Becton-Dickinson and Company of Franklin, Lakes, N.J.
Since the buffy coat layers are very thin, the buffy coat is often expanded in the capillary tube for more accurate visual or optical measurement by placing a plastic cylinder, or float, into the tube. The float has a density less than that of red blood cells (approximately 1.090 g/ml) and greater than that of plasma (approximately 1.028 g/ml) and occupies nearly all of the cross-sectional area of the tube. The volume-occupying float, therefore, generally rests on the packed red blood cell layer and expands the axial length of the buffy coat layers in the tube for easier and more accurate measurement.
There exists a need in the art for an improved sample tube and float system and method for separating blood and/or identifying circulating cancer and/or other rare cells, organisms or particulates or objects (i.e., stem cells, cell fragments, virally-infected cells, trypanosomes, etc.) in the buffy coat or other layers in a blood sample. However, the number of cells expected to be typically present in the buffy coat is very low relative to the volume of blood, for example, in the range of about 1–100 cells per millimeter of blood, thus making the measurement difficult, particularly with the very small sample sizes employed with the conventional QBC capillary tubes and floats.
The present invention contemplates a new and improved blood separation assembly and method that overcome the above-referenced problems and others.